The present invention relates to a hybrid for providing a pair of 90 degree phase shifted signals to a pair of antennas, and more particularly, to such a hybrid that is arranged for compactness and is broadband.
Interest has grown in using circular polarization (CP) for television broadcasting in order to reduce ghosts and fading. One way of achieving CP is to use a pair of dipoles disposed at a right angle with respect to each other and fed with equal currents and in phase quadrature, i.e., with 90 degrees of phase shift therebetween.
In order to achieve the required quadrature phase shift, a four port hybrid comprising a line having a length of one quarter wavelength at a selected center frequency can be used. Such a hybrid has an input port, a port terminated in its characteristic impedance, and two unbalanced output ports at opposite ends thereof. However, this type of hybrid is bulky due to the length of said line and requires flexible coaxial cables to extend from the two output ports, which are at opposite sides of said hybrid, through tubes comprising baluns, which baluns are required to feed the dipoles, since the dipoles are a balanced load.
Another type of hybrid used lumped elements, as shown in FIG. 1. A coaxial cable is connected with its center conductor connected to terminal 10 and its outer shield to terminal 12. Resistors 14 and 16 represent the load resistance R presented by each of a pair of dipoles respectively, having currents I.sub.1 and I.sub.2, respectively, flowing therein. If, at a selected center frequency, the reactances of inductor 18 and capacitor 20 are made equal to +jR and -jR, respectively, then I.sub.1 and I.sub.2 are equal in magnitude and in phase quadrature. Further, the input impedance at terminals 10 and 12 is just R. However, this type of hybrid has stray fields from components 18 and 20 to each other and to the shielded box that normally surrounds them. This can make the currents unequal and not in phase quadrature. In addition, the inductor 18, when designed for use at a high frequency, e.g., high band VHF channels 7-13, is small and therefore difficult to reproduce.
Yet another type of hybrid uses short and open circuited transmission lines to provide inductance and capacitance respectively, to replace inductor 18 and capacitor 20, respectively, of FIG. 1. However, when these transmission lines have a characteristic impedance of 50 ohms, it has been found that the resulting bandwidth of the driven antenna as determined by its axial ratio (AR) is less than that of the lumped element hybrid of FIG. 1. By "AR" is meant the ratio of maximum to minimum linear fields strengths.
It is, therefore, desirable to provide a hybrid that is compact, has a broad bandwidth, and is easy to manufacture to the required tolerances so that the output signals are of substantially equal amplitude and in phase quadrature.